Ferromagnetic limiter



March 19, 1963 E. STERN FERROMAGNETIC LIMITER Filed Nov. 22. 1960 2Sheets-Sheet l ou"r RES SUB

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" INVENTORI ERNEST STERN,

BY m1 HIS AGENT.

March 19, 1963 E. STERN 3,

FERROMAGNETIC LIMITER Filed Nov. 22. 1960 2 sheets sheet 2 3 L0 (ATSUBSIDIARY RESONANCE) P LOG PIN )l asA W-u ase- INVENTORI ERNEST STERNBY J. I

HIS AGENT.

United States Patent 3,082,383 FERROMAGNETIC LIMITER Ernest Stern,Liverpool, N.Y., assignor to General Electric Company, a corporation ofNew York Filed Nov. 22, 1960, Ser. No. 70,943 '7 Claims. (Ci. 333-1.]l)

This invention relates to microwave solid state limiters employing aferromagnetic limiter element having a low limiting threshold and ashort response time.

It is well known that ferromagnetic materials exhibit a precessionphenomenon. This phenomenon, as other magnetic properties offerromagnetic materials, is usually described in terms of unpairedelectron spins occurring in some atoms such as iron and nickel whichproduce a net magnetic moment. The unpaired electron spin model leads tothe precession phenomenon which may be considered an analog of themechanical gyroscope. That is, in the presence of a constant magneticfield, the axis of rotation of the electron precesses around thedirection of the magnetic field. In a crystal lattice, the precessionalphenomenon is coupled between neighboring atoms causing spin Waves,which are propagated throughout the lattice. These spin waves may be oflong or short wavelength, varying from two lattice lengths to the fulldimensions of the member and randomly distributed in the member.

The precession phenomenon and the accompanying spin waves lead tocomplex efiects. One such effect is that ferromagnetic materials exhibita nonlinear dissipative response to electromagnetic waves which excitethe spin waves. This nonlinear effect is such that for electromagneticwaves below a given amplitude, there Will be little signal attenuation,but for signals exceeding this given amplitude, there will be asubstantially constant amplitude output. An example of a ferromagneticlimiter operating upon this principle is disclosed in the IRETransactions on Microwave Theory and Techniques, January 1959(Characteristics of Ferrite Microwave limiters, by G. S. Uebele) inwhich a slab of ferrite is piaced in the microwave magnetic field of awaveguide.

The prior ferromagnetic limiters have had inherent limitations whichhave made practical applications difficult. One of the most seriousproblems has been the spike leakage which the ferromagnetic limiterspass when an applied pulsed signal has a rapid rise time and a magnitudewhich exceeds the limiting threshold. The leakage spike approaches theinput signal amplitude and has a time duration typically on the order ofto 10- seconds. Qualitatively, this leakage arises from the finite timerequired to develop excitation of the ferromagnetic material, a timewhich increases with the Q of the material. This is inherent in thenature of the gyromagnetic effect. The energy passed during thistransient effect can be reduced by increasing the linewidth, i.e.lowering the Q of the ferromagnetic material since the spike durationtime is inversely proportional to the linewidth. Unfortunately, ferritesexhibiting increased linewidth also exhibit increased limitingthresholds. For a signal substantially coinciding with the subsidiaryresonance frequency, the limiting threshold varies directly with thelinewidth. This limitation is in addition to that imposed by the directproportionality relationship between the microwave magnetic field in theferromagnetic material and the square root of the incident signal powerwhich limits the portion of the incident energy operated upon.

It is an object of this invention to provide an improved microwavelimiter employing a ferromagnetic medium in a configuration whichintensifies the microwave signal field.

It is a further object of this invention to provide a microwave limiteremploying a ferromagnetic medium with a low limiting threshold and aleakage spike of shortened duration.

3,682,383 Patented Mar. 19, 1%53 Briefly stated, in accordance with oneaspect of the invention, a ferromagnetic limiter element operating in aDC. magnetic field is arranged to limit microwave signals in a sectionof waveguide. The limiter element is com prised of a conductive filamentterminating in enlarged end portions and surrounded with a body offerromagnetic material. The end portions of the limiter element areshaped to prevent breakdown and maximize the filament current. Theferromagnetic body of the limiter element is dimensioned to give ademagnetization geometry such as to provide a subsidiary resonance atthe signal frequency near to but separated from the main resonance. Thelimiter element is supported at a point of maximum electric signal fieldand in alignment with the direction of the signal field. The magneticfield is oriented parallel to the axis of the ferromagnetic limiterelement and is adjusted to bring the subsidiary resonance frequency incoincidence with the signal frequency.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawings and its scope will bepointed out in the appended claims. 7

FIGURE 1 is a block diagram of a suitable limiter combinationincorporating a limiter element constructed in accordance with theapplicants invention.

FIGURE 2 is a cross section of a limiter element constructed inaccordance with the applicants invention.

FIGURE 3 is a diagram of the equivalent circuit of the FIGURE 2 limiterelement.

FIGURE 4 is a graph of the power output to power input ratio as afunction of the constant magnetic field, H applied to the limiter.

FIGURE 5 is a graph of output power as a function of time for a limiterincorporating the invention.

FIGURE 6 is a graph of the log of power output as a function of the logof power input for a novel limiter incorporating one, two and threelimiter elements.

FIGURE 7 is an example of another limiter configuration.

A typical circuit environment for a limiter constructed in accordancewith the disclosed invention is illustrated in FIGURE 1 in which thelimiter serves as a detector protector in an X-band radar system inaddition to the usual duplexer. A conventional circulator 1 providingcounterclockwise connection between the ports is shown. It provides aconnection between a duplexer 2 and a radar receiver 3 through a pathincorporating the limiter circuit in a radar system requiring thelimiting function. The circulator may be constructed in accordance withthe disclosure found in the Journal of Applied Physics, supplement tovolume 30, No. 4, April 1959 Y circulator, by H. N. Chait and T. R.Curry). The input signal, before it is transmitted to the output port,is connected through the limiter and through a quarter wavelength delayline 4 to a short 6 where it is reflected back through the circulator 1.The limiter serves to attenuate the input signals above a giventhreshold power amplitude. Suitable dimensions for the waveguideinterconnections are one inch by one half inch in which the signals arepropagated in a TE mode.

The circuit configuration of FIGURE 1 produces a standing wave as seenby the limiter 4. This feature is not essential to the operation of thelimiter 4 per se. However, it is a convenience which presents the signala second time for limiting action and with the reflecting shortarrangement, it is easier to design the limiter. This is because therequirement that the limiter impedance match the waveguide impedance isrelaxed.

FIGURE 2 illustrates a longitudinal cross section in elevation of apreferred embodiment of the limiter 4 of FIGURE 1. A waveguide section21 is arranged to support a limiter element 22 at a point of maximumelectric field intensity and in alignment therewith. The limiter elementincludes a conductive filament 23 terminating at each end in enlarged,disk-shaped conductive portions 24 which reduce the density of chargeaccumulation resulting from currents induced by a signal in thewaveguide. The filament is surrounded by a-ferromagnetic sheath 25extending the length of the filament and preferably contiguoustherewith. The fabrication and properties of the ferromagnetic sheathwill be described below. The limiter element is supported by quartzstuds 27 cemented to the waveguide walls and to the disks 24. Thelimiter element is subjected to a magnetic bias field by pole pieces 28,29 which are orieinted to create a field aligned with the limiterelement. The bias field strength is adjusted to produce correspondenceof the ferrite subsidiary resonance frequency with that of the signalfrequency.

The limiter element 22 serves to concentrate the energy of the waveguidesignal into the ferromagnetic material 25 which produces an improvedcoupling between the signal and the ferromagnetic material. The couplingresults from the alternating current induced in the limiter filament bythe electric field of the signal, which in turn creates a magnetic fieldacting upon the ferromagnetic material. A filament to concentrate amicrowave signal has been disclosed by D. Rodbell in the Journal ofApplied Physics, volume 30, No. 4, November 1959 (Microwave MagneticField Near a Conducting Perturbation) and has been described and claimedin an application for US. Letters Patent entitled Magnetic MicrowaveDevice, Serial No. 65,085, filed October 26, 1960, by Donald S. Rodbelland assigned to the assignee of the present invention. When the limiterelement configuration consists of a good conductor surrounded byferromagnetic material, the result can be a concentration of theeffective microwave field by several orders of magnitude. Thisconcentration makes it possible to reduce the threshold signal powerrequired to excite the subsidiary resonance and permits the use of abroader linewidth material.

The positioning of the limiter element in the waveguide is not overlycritical, but variations from the optimum position will increase thethreshold level for limiting bydecreasing the current induced in theconductive filament 23. For example, misalignment of the limiterfilament with the direction of the electric field by an angle willreduce the threshold level by a cos 2 0 factor. Displacement from theposition of maximum electric field will result in a similar reduction ofthe threshold level.

As shown in FIGURE 2, one type of limiter element utilizes a ferrite asthe ferromagnetic material 25 formed on the filament conductor 23. Onemethod of applying the ferrite is to imbed a fine wire in the unfiredferrite material. The techniques of either dry pressing the ferrite in amold in which a Wire is centered or extruding the ferrite around a wireinserted in an extrusion die are suitable. After the combination is thusformed, it is fired at sufliciently high temperatures to sinter theferrite. It is necessary to use metals such as platinum for the filamentwire to avoid melting and reaction of the wire with the ferrite at thehigh temperatures in firing.

The ferrite type of limiter element must be formed with the proper shapeto produce the desired limiting efiect. That is, the subsidiarygyromagnetic resonance of the ferrite body must be tuned to the signalfrequency. FIG- URE 4 is a graph of the ratio of power output to powerinput against bias field for an input signal of a given frequency andpower amplitude above the limiting threshold. This curve ischaracterized by two resonance lines, the main resonance line at H andthe lower, subsidiary resonance line at H The main resonance line isessentially determined by the properties of the material and theexternal dimensions thereof. Within the range of signal power where thesubsidiary resonance effects occur, the main resonance represents a lossfactor which is not of utility in a limiter application. The subsidiaryresonance line, however, only appears for input signals above thelimiting threshold and provides the limiting mechanism relied on in thepresent invention. The position of the subsidiary resonance linerelative to the main resonance line is a variable factor. The variationobtainable extends from coincidence between the resonances to a positionof substantial displacement of the subsidiary resonance line below themainresonance line as shown in FIGURE 4. This separation factor isdetermined by the demagnetization effects in the ferromagnetic materialand for a given material is a function of the geometry of the body asexplained hereinafter.

The optimum design of a limiter element involves consideration ofseveral interrelated factors. These factors include maintainingproportions of the limiter element dimensions to a satisfactoryimpedance match of the conductive structure with the waveguide; theselection of a material with the desired magnetic properties; shapingthe body of ferromagnetic material to produce the desired smalldisplacement of the subsidiary resonance from the main gyromagneticresonance and obtaining the desired relation between the amplitude ofthe signal in the waveguide and the amplitude of the magnetic fieldproduced by the conductive filament so that limiting action isintroduced at the desired signal level.

The first consideration in designing a limiter element is the selectionof proper physical dimensions to produce satisfactory coupling of thesignal from the waveguide to the limiter element. It has been found thatthe limiter element, considered as a lumped circuit element must presenta good impedance match to the waveguide to avoid substantial reflectionof the signal.

The equivalent circuitof the limiter element and Waveguide isillustrated in FIGURE 3. Capacitance appears between each of themetalized ends of the limiter element 24A and 24'13, and between each ofthe metalized ends and the walls of the waveguide 21'. The currentbearing filament presents an inductance 23' and the ferromagneticabsorption in the ferrite approximates a nonlinear resistance 25'.Accordingly, the limiter appears as a parallel resonant circuit inseries with a pair of capacitances. It is essential that this circuitresonate in the region of the ,signal frequency to achieve the desiredcoupling of the signal to the limiter element.

The underlying physical phenomena giving rise to the lossy element 25 ofthis parallel resonance circuit will now be discussed.

For optimum sensitivity, the subsidiary resonance frequency should bepositioned as close to the main resonance as possible. The thresholdlevel for the subsidiary resonance decreases as the subsidiary resonanceis removed from the main resonance. However, if the signal frequencyshould fall within the main resonance line width, the signal will see anadditional fixed attenuation factor which would appear as an undesiredlimiter insertion loss. Accordingly, the limiter element is designed tohave a small but clearly delineated displacement between the tworesonance peaks. In terms of the applied magnetic field intensity, thedisplacement is typically twice the main resonance linewidth 2AH.

For a cylinder, the main resonance and subsidiary resonance arerespectively determined as follows:

where H is the main resonance, H is the subsidiary resonance, N, is thedemagnetization factor, to is the signal frequency, 41rM is thesaturation magnetization and 'y is the gyromagnetic constant. To providethe desired ZAH displacement, Equations 1 and 2 are combined to producethe requirement:

(3) Ni fall 1g,

From this expression a solution for N can be obtained. From the articleby Osborn in the Physical Review, volume 67, June 1945 (DemagnetizingEffects of the General Ellipsoid), the required ratio of diameter tolength can be obtained.

The absolute dimensions may vary over a substantial range, having, ofcourse, a maximum dimension limited by the size of the transmissionline.

Expressions 1 and 2, which apply only to the solution for a cylinder,also are of the same general nature as those of the simple geometricalshapes.

The objective of a limiter is to remove all signal power above a giventhreshold signal power level. In the limiter element configuration ofFIGURE 2, the magnetic field operating on the ferromagnetic material isnot primarily the waveguide field at the signal frequency, but ratherthe field produced by the current in the conductive structure of thelimiter. Since the conductive structure con-- figuration, whichdetermines the filament current is subject to design, the relationbetween waveguide field amplitude and the amplitude of the magneticfield acting upon the ferromagnetic material can be controlled by thisdesign.

For example, in the FIGURE 2 limiter element, the current is primarilydetermined by th conductive filament diameter, the impedance of theelement, and the area of the disk shaped metallized end portions 24. Thecharge induced in the ends is if the field terminating on the limiterelement end portions is equal to the electric field intensity E of theempty waveguide at the signal frequency, where A is the area of the endportions and s is the permittivity constant.

Accordingly, from the Biot-Savart law, the magnetic field produced atthe filament surface is aproximately where w is the signal frequency, ris the filament radius.

The particular application will specify the threshold power, the spikeleakage and signal frequency. A material is selected that will providethe minimum h consistent with the spike leakage requirements. Thedimensions of the ferrite element, the wire diameter and disk diameterare then chosen to produce the specified threshold power.

The response of the novel limiter element is illustrated in FIGURE 6.For a step input signal pulse shown in dashed lines the output power isshown by the solid line. Initially, the output amplitude essentiallyequals the high signal pulse but it rapidly falls to a plateausubstantially at the threshold power amplitude, P This initial spikeresponse as pointed out earlier is an inherent feature of the resonancemechanism of the ferromagnetic material. Although it cannot beeliminated, it can be alleviated by reducing the spike width to minimizethe energy content of the spike.

The spike duration is equal to where 'y is the gyromagnetic constant ofthe material and AH is the spin wave linewidth. By selecting a materialhaving an appropriate linewidth, the specified spike duration can beachieved.

The required threshold of microwave field intensity is determinedtheoretically by the following relation:

An appropriate material and shape can be selected to provide a lowthreshold field.

FIGURE 5 is a graph of the log of output power response of the limiteragainst the log of input signal power at the subsidiary resonancefrequency. Curve 41 is the response of a single limiter element andcurves 42 and 43 are for two and three limiter element configurations,respectively. As all of these curves show, the response of the limiteris comprised of three regions. The first region is characterized by alinear relation between input and output power with a small differentialdue to insertion loss. This region is followed by a second region inwhich there is no further increase in output power as the input powerincreases. This is the limiting region which typically extends over athirty decibel range. Above this range, is a region of no limiting wherethe lossy spin wave mechanism is saturated. Here, for each increment ofinput power there is an equal increment of output power. The efiect of aplurality of limiter elements is the multiplication of the limitingrange. The onset of limiting action remains fixed, but the region 'oflimiting is extended to twice the single element limiting range for atwo element configuration as shown by curve 42, and the limiting rangeis tripled for three limiter elements as shown by curve 43. Thismultiplication effect holds when the ele ments are connected in series.

The conductive disk-shaped end portions 24 of the limiter elementillustrated in FIGURE 2 are easily formed elements for chargeaccumulation. These portions are produced by the application of silverpaint over the end portions of the ferrite resulting in the conductivedisks. One of the primary criteria in the design of the end portions isthe prevention of break down which is determined by factors such as thedielectric in the waveguide and the signal strength. The enlargedconductive end portions also produce the important result that limiterelement current is increased by the enhanced charge storage. The desiredsignal field intensity in the material is proportional to this currentso that this factor is usually critical. Clearly, shapes other than thedisk-shaped configuration such as tear-shaped end portions can reducethe charge density. Another approach is to ground the ends of thelimiter element to the waveguide walls, but the enlarged end portionarrangement can produce larger currents than the grounded limiterelement arrangement.

Another type of limiter element is comprised of a film of Permalloy asthe ferromagnetic material 26 formed on the filament conductor 23. Themethod of film formation is conveniently electrodeposition in accordancewith the process disclosed in the 44th Proceedings of the AmericanElectroplaters Society, 1957 (Further Studies on Nickel-Iron AlloyElectrodeposits, by I. W. Wolf) and the 43rd Proceedings of the AmericanElectroplaters Society, 1956 (Nickel-Iron Alloy Electrodeposits forMagnetic Shielding, by I. W. Wolf and V. P. McConnell). By this method,films on the order of one thousand angstroms in thickness are formedusing a one mil diameter gold wire as the fihn substrate and filamentconductor.

A second exemplary limiter configuration is illustrated in FIGURE 7. Inthis configuration, a pair of limiter elements are arranged oposite oneanother on the walls of a waveguide 31. A pair of conductive filaments33A and 33B are positioned in alignment and in contact with thewaveguide walls. Surrounding the filaments 33A and 33B are coaxialcylindrical bodies of ferromagnetic ceramic 35A and 35B, respectively.The choice of materials and dimensions of the limiter elements aredetermined by the same considerations as the FIGURE 2 embodiment. Thelimiter elements are separated by an insulator 36 selected to providemechanical and electrical stability to this configuration. Between eachcylinder of ferromagnetic material 35A and 35B and the insulator 36 isplaced a conductor 34in electrical contact with corresponding conductivefilaments 33A and 33B. These 7 conductors 34 serve the same function asthe conductive portions 24 in the FIGURE 3 limiter element and areconveniently produced by the application of silver paint on thecylinders 35A and 35B. The FIGURE 7 embodiment also operates in the samemanner as that described for the FIGURE 2. embodiment. The electricfield of a signal in the waveguide sets up oscillating currents in thefilaments 33A and 33B which are dissipated by magnetic coupling to thelossy spin waves in the ferrite 35A and 353 for signals exceeding thethreshold limiting amplitude.

The ferrite materials which have particular advantage in the presentinvention are those having an inreased subsidiary resonance line widthsince they are characterized by a shorter response time. An adverseproperty concurrently shared by these favored materials and offset inaccordance with the present invention is that the required fieldintensities must be considerably higher before this broadened linewidthcapability comes into play. Suitable material for use at X-band arethose having. linewidths of large AH containing in their latticestructure fast relaxers such as the cobalt ion.

The ferrite limiter designed in accordance with the disclosed inventionhas as a principal advantage the reduction of spike duration frombetween and 10* to 10- seconds. The selected configuration, employing aconductive filament, concentrates the signal energy producing alocalized magnetic field in the ferrite substantially exceeding themagnetic field strength normally applied to the ferrite. This makespossible the use of'those ferromagnetic materials having broader spinabsorption line propertes which require higher field concentrations. Asexplained earlier, the ability to use broader linewidth materialsshortens the response time, and thus reduces the duration of the spikeleakage.

While particular embodiments of the invention have been shown anddescribed, it should be understood that the invention is not limitedthereto and it is intended in the appended claims to claim all suchvariations as fall within the true spirit of the present invention.

What is claimed is:

l. A solid sate limiter comprising: a signal transmitting wave guide; aconductor positioned in said waveguide subject to the oscillatoryelectric field of the waveguide to produce current oscillations thereingiving rise to an oscillatory magnetic field having an amplitudeexceeding that of the waveguide magnetic field strength, a body offerromagnetic material positioned in said waveguide proximate saidconductor for substantial flux linkage with said oscillatory magneticfield, said body exhibiting a subsidiary resonance separated from themain gyromagnetic resonance frequency; and magnetic means arranged toproduce a constant magnetic bias field in said body of ferromagneticmaterial of a magnitude such as to make the subsidiary resonancecoincident with the frequency of the waveguide signal.

2. The limiter of claim 1 wherein: at least one end of said conductor isconductively connected to said waveguide.

3. A solid state limiter comprising: a signal transmitting waveguide; aconductive filament positioned in said waveguide subject to theoscillatory electric field of the waveguide to produce currentoscillations therein giving rise to an oscillatory magnetic field havingan amplitude exceeding that of the waveguide magnetic field strength; asubstantially cylindrical body of ferromagnetic material surnoundingsaid filament and proximate thereto and dimensioned to exhibit asubsidiary resonance separated from the main gyromagnetic resonancefrequency; and magnetic means arranged to produce a constant magneticbias field in said body of ferromagnetic material of a magnitude such asto make the subsidiary resonance coincident with the frequency of thewaveguide signal.

4. The solid state limiter of claim 3 further including: a pair ofconductive charge storage elements, each element being connected to oneend of said conductive filament.

5. The solid state limiter of claim 3 further including: a pair ofconductive elements formed on the end surfaces of said cylinder inelectrical contact with said filament, said elements being shaped toprevent electrical breakdown between said filament and said waveguideand to produce increased current oscillations in said filament.

6. A solid state limiter comprising: a signal transmitting waveguide; aconductive filament positioned in said waveguide subject to variationsin the electric field of the waveguide signal to produce currentoscillations therein giving rise to an oscillatory magnetic field havingan amplitude exceeding that of the waveguide signal magnetic fieldstrength; a substantially cylindrical body of ferromagnetic materialhaving a broad spin absorption line property surrounding said filamentand proximate thereto and dimensioned to exhibit a subsidiary resonanceremoved from the main gyromagnetic resonance frequency; and magneticmeans arranged to produce a constant magnetic bias field in said body offerromagnetic material of a magnitude such as to make the subsidiaryresonance coincident with the frequency of the waveguide signal.

7. A limiter comprising: an isolating microwave element having first,second and third ports, said element having the property that an inputsignal received at one of said ports is transmitted only to said port ofsucceeding number; a waveguide element connected to one of said ports;reflecting means connected to said waveguide element arranged to producea standing wave pattern therein; and a limiter element positioned insaid waveguide element, said limiter element being comprised of a filamentary conductor producing current oscillations and a body offerromagnetic material exhibiting a subsidiary resonance separated fromthe main gyromagnetic resonance frequency, said body being proximatesaid conductor for substantial flux linkage with the magnetic fieldaccompanying said current oscillation.

References Cited in the file of this patent UNITED STATES PATENTS

1. A SOLID STATE LIMITER COMPRISING: A SIGNAL TRANSMITTING WAVE GUIDE; ACONDUCTOR POSITIONED IN SAID WAVEGUIDE SUBJECT TO THE OSCILLATORYELECTRIC FIELD OF THE WAVEGUIDE TO PRODUCE CURRENT OSCILLATIONS THEREINGIVING RISE TO AN OSCILLATORY MAGNETIC FIELD HAVING AN AMPLITUDEEXCEEDING THAT OF THE WAVEGUIDE MAGNETIC FIELD STRENGTH, A BODY OFFERROMAGNETIC MATERIAL POSITIONED IN SAID WAVEGUIDE PROXIMATE SAIDCONDUCTOR FOR SUBSTANTIAL FLUX LINKAGE WITH SAID OSCILLATORY MAGNETICFIELD, SAID BODY EXHIBITING A SUBSIDIARY RESONANCE SEPARATED FROM THEMAIN GYROMAGNETIC RESONANCE FREQUENCY; AND MAGNETIC MEANS ARRANGED TOPRODUCE A CONSTANT MAGNETIC BIAS FIELD IN SAID BODY OF FERROMAGNETICMATERIAL OF A MAGNITUDE SUCH AS TO MAKE THE SUBSIDIARY RESONANCECOINCIDENT WITH THE FREQUENCY OF THE WAVEGUIDE SIGNAL.